This invention relates to liquid metal cooled fast breeder nuclear reactors.
In one construction of liquid metal cooled fast breeder nuclear reactor wherein the reactor core, intermediate heat exchangers and liquid metal pumps are immersed in a pool of coolant such as sodium, the intermediate heat exchangers are suspended from the roof of the construction. Ducting in the form of a core tank or core shroud, interconnected with pods housing the intermediate exchangers, is provided for directing coolant from the core over the heat exchanger tubes thence back to the main pool of liquid metal. Seals are provided between the intermediate heat exchanger shells and the walls of their pods to prevent liquid metal flow by-passing the heat exchanger tube bundles. As the heat exchangers must be withdrawable for servicing and because linear differential thermal expansion of the heat exchanger and its pod must be accommodated, the seals hitherto have been of the sliding kind generally known as piston ring type seals. There are several disadvantages with this kind of seal when used for the hereinbefore described purpose, for example, sealing is not absolute, the metal-to-metal seal causes wear and fretting by rubbing and vibration, which eventually could lead to seisure and the seal could become jammed by the deposition of impurities in the liquid metal coolant. Yet another difficulty resides in the need to accommodate lateral thermal expansion of the ducting including the core tank and pods. Hitherto some expansion has been accommodated by expansible bellows pairs in the interconnections or, alternatively, by local deformations of the core tank and pods. Expansible bellows need to be very flexible and hence constitute a weak section of the ducting, local deformations of the ducting give rise to high stress levels which could lead to premature failure of the ducting.